Add Delete Flag to Paths ignore mask

Summary:
When `arc diff` runs unit tests it uses all of the affected files as the base array of paths. These files may have been deleted. If the deleted file fits the test criteria `unit` will try and run the test and in some cases fail.

An example of this fataling is with PHPUnit;

> Fatal error: Uncaught exception 'Exception' with message 'JSON report file is em
pty, it probably means that phpunit failed to run tests. Try running arc unit wi
th --trace option and then run generated phpunit command yourself, you might get
 the answer.' in C:\Websites\facebook\arcanist\src\unit\engine\PhpunitResultPars
er.php on line 156

Test Plan: Re-ran the tests that were causing issues with `arc unit --rev HEAD^ --trace`

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, aurelijus

Differential Revision: https://secure.phabricator.com/D6246